Thousands of demonstrators gathered in Buenos Aires on Friday to march against the G20's economic policies as the world's leading industrialized nations opened their annual summit in the Argentine capital.

But the protesters were unlikely to get anywhere near the leaders of the United States, Russia, China and other global powers gathered for a meeting whose agenda was expected to be dominated by the U.S.-China trade war.

Police, coast guard and border patrols cordoned off a 12-square-kilometre (5-square-mile) area around the riverside Costa Salguero convention centre, where the summit is being held. Cargo traffic on La Plata River was shut down for the event.

The march, organized by a coalition of labour unions and rights groups, was being held about 5 km (3 miles) away from the summit and was due to start at 3 pm (1800 GMT).

Buenos Aires was largely locked down. Public transportation was suspended and hundreds of intersections blocked to control traffic and crowds. Friday was declared a national bank holiday and Argentina's centre-right government suggested people leave town for the summit.

Those measures made it hard for activists to get to the march.

"The government imposed prohibitive terms," said Beverly Keene, march coordinator and head of Jubileo Sur-Dialogo 2000, a group that campaigns for Latin American debt reform.

Protests are common at such large global gatherings, with disparate groups of demonstrators often finding common ground in protesting against what they see as the rich world's neglect of the poor and marginalized.

Police were on high alert after a melee at a Buenos Aires soccer stadium derailed the final leg of South America's main club tournament on Nov. 24.

President Mauricio Macri said the incident, which raised questions about the effectiveness of Argentine security forces, "an embarrassment" and called for tougher law enforcement.

December is usually the month when protests over Argentina's chronically troubled economy turn violent. With inflation at a dizzying 45 percent and the economy contracting, popular unrest could be an additional factor to worry security services.

RESTRICTED ROUTE

The Security Ministry restricted the route for the anti-G20 march to a 2.5-km (1.55 mile) stretch from the city's main street, Avenida 9 de Julio, to the country's Capitol building.

Keene had proposed a longer route that would have included Buenos Aires' central obelisk and Plaza de Mayo behind the Casa Rosada presidential palace, which is Argentina's traditional protest venue.

Last year Germany hosted the G20 summit in the centre of the northern port city of Hamburg. At that meeting, police struggled to contain black-clad anti-capitalist militants who torched cars, looted shops and hurled Molotov cocktails while tens of thousands more people demonstrated peacefully.